Computer interceptor proximity fuze

ABSTRACT

A pulsed radar proximity fuze for mounting into an interceptor missile for attacking a flying target comprising a computerized pulsed radar for measurement of range to the target, computation of the time at which the target will be at the point of closest approach to the interceptor missile, and computation of the miss distance between the interceptor missile and the target. The fuze from this information which is stored and computed by a computer, controls the instance of detonation of the missile warhead to achieve maximum lethal effect. The computer calculates tau, the ratio of the separation between the interceptor missile and the target to the relative velocity of the target with respect to the interceptor. When the absolute value of tau is an extremum, tau being a time value, the time remaining to go to the point of closest approach from the occurrence of the extremum value of tau is one half of the absolute value of that extremum value. A clock in the computer counts the time from extremum tau to point of closest approach and appropriately issues a trigger signal to the detonator. By storing the value of the fragment velocity, the computer may be set to allow for this value and thus institute a time delay such that such fragments encounter the target at the point of closest approach.

United States Patent [191 Krupen [4 Nov. 26, 1974 COMPUTER INTERCEPTOR PROXIMITY FUZE Inventor: Philip Krupen, Silver Spring, Md.

Assignee: The United States of America as represented by the Secretary of the Army, Washington, DC.

Filed: Dec. 4, 1973 App]. No.: 421,550

US. Cl. l02/70.2 P, 343/7 PF Int. Cl. F42c 13/04 Field of Search l02/70.2 P; 343/7 PF, 3,

References Cited UNITED STATES PATENTS 6/1972 Ziemba l02/70.2 P 7/l973 Dick 102/702 P OTHER PUBLICATIONS Frye et al., Aircraft Collision Avoidance Systems, IEEE Spectrum, January 1966, pages 72-80.

[5 7] ABSTRACT A pulsed radar proximity fuze for mounting into an interceptor missile for attacking a flying-target comprising a computerized pulsed radar for measurement of range to the target, computation of the time at which the target will be at the point of closest approach to the interceptor missile, and computation of the miss distance between the interceptor missile and the target. The fuze from this information which is stored and computed by a computer, controls the instance of detonation of the missile warhead to achieve maximum lethal effect. The computer calculates tau, the ratio of the separation between the interceptor missile and the target to the relative velocity of the target with respect to the interceptor. When the absolute value of tau is an extremum, tau being a time value, the time remaining to go to the point of closest approach from the occurrence of the extremum value of tau is one half of the absolute value of that extremum value. A clock in the computer counts the time from extremum tau to point of closest approach and appropriately issues a trigger signal to the detonator. By storing the value of the fragment velocity, the computer may be set to allow for this value and thus institute a time delay such that such fragments encounter the target at the point of closest approach.

4 Claims, 4 Drawing Figures 0 I- 1 1 l MODULATOR MAGNETRON Am n2 I 94 98J 91 on IP90, T

I I 15 9 L0 MIXER 2 1? 96) 95) i RMGE MEASURE \noEo -DETECTOR L MEMORY =52 R NrmN DWlDE STORAGE T D\\/lDE BY'Z. l DELAY BYim l J r J J- it =tt/ 34 T] 'c A l COMPARE E i I I 1 tt (1 3 '73 l l 35 L -l. -J 7 so oeioNMoR 1 COMPUTER INTERCEPTOR PROXIMITY FUZE RIGHTS OF THE GOVERNMENT BACKGROUND OF THE INVENTION The principles of this invention are related to instrumentation for aircraft collision avoidance. An article on the art of aircraft collision avoidance as related to this invention has been written by E. O. Frye and O. E. Killham and entitled Aircraft Collision Avoidance Systems," IEEE SPECTRUM, January 1966, pages 72-80.

Usual proximity fuzes are those in which the action is created within the fuze from characteristics other than actual contact of lapse time characteristics. Proximity fuzes are sometimes referred to as influence fuzes and usually initiate the munition when they sense that they are in the proximity of the target. This action is particularly effective in use against personnel, light ground targets, aircraft, and super structures of ships.

Different fuzes are assigned specific tasks. Some are assigned to detonate as they approach their target, others are expected to detonate upon impacting the target; and still others are expected to detonate only after penetrating the target. In some cases, it is desired that the fuze provide for optional actions. Some fuzes are required to destroy the munition if no target is sensed within a given time interval or flight distance.

In every instance, the fuze must first sense the target at the proper time or distance so that its subsequent actions may be initiated. This problem is usually solved in one of these ways: (1 sensing by contact between munition and target, (2) influence sensing with no contact between munition and target, and (3) presetting in which the functioning delay of fuzes is set before launching or implacement. This invention is specifically concerned with the second item of the above solutions.

The influence sensing type of fuze results in detonation of the bursting charge in the vicinity of the target. This does not necessarilymean that the fuze will explode at the most lethal separation distance between the fuze and the target. It is useful to spray fragments of the missile onto the target rather than have direct impact for a number of reasons. Primarily in the case of missiles attacking aircraft the probability of a particular fragment hitting an aircraft is much greater than a direct hit by a missile. Filling the air around an aircraft with fragments has the net effect of enlarging the target.

The leading type ofinfluence sensing is the proximity fuze of the radio type. In this example the proximity fuze contains a continuous wave transmitter, an antenna, and a receiver. When the emitted waves strike a target, some of the energy is reflected back to the antenna. Because of the relative motion between fuzes and target, reflected wave frequency differs from the original emitted frequency and the difference (known as the Doppler or beat frequency) is generated in the antenna and amplified in the receiver. When the signal reaches a certain value, an electric detonator is initiated that in turn functions the explosive train.

A recent refinement of the influence sensing type fuze has come to be in the area of surface-to-air guided missiles. The missile must sense the target both to follow it and to initiate the fuze action. Such systems as the foregoing do not provide for detonation at the point of closest approach or the minimum separation distance between the fuze and the target. Moreover, such systems as the foregoing do not provide for detonation of the fuze at such a time as to allow the fragmentation to intercept the target at the point of closest approach.

It is accordingly an object of this invention to provide a fuze which utilizes pulsed radar techniques, logic circuitry, and electronic computer memory devices in order that the fuze may be detonated at the minimum separation distance between the fuze and the target.

It is also an object of this invention to provide a pulsed radar fuze system with a computer that will enable the fuze to detonate itself at such a point in relationship to the target such that the fragmentation from the fuze encounters the target at the point of closest approach.

It is another object of this invention to provide an interceptor fuze with a computer for use in a missile for computation of the time function tau, where tau is the ratio of the separation distance to the associated range rate of the target with respect to the interceptor, such that at the instant the time function tau is an extremum a counter is initiated which causes the fuze to explode at the point of closest approach.

It is still a further object of this invention to provide a method for detonating a fuze at the point of closest approach to a target.

Further objects of this invention and the entire scope of this invention will be fully apparent in the following detailed description and the appended claims.

SUMMARY OF THE INVENTION In combination with a pulsed radar proximity fuze of the type wherein the separation distance between said fuze and a target iscomputed and said fuze is detonated when a predetermined separation distance is detected, a system and method for computing from measurements of range to the target, the exact time that the target will be at the point of closest approach to the interceptor, and what the miss distance will be. This system and method is used to control the point in time at which to detonate the missile warhead in order to achieve maximum lethal effect. A pulsed radar seeker follows the target and measures the range by well known radar pulse techniques. This information is differentiated to obtain dr/dt, and then the ratio tau. r+dr/dt is computed. Tau is a time function that passes through a mathematical extremum. Its occurrence is detected by an extremum detector. The remaining time to go to the point of closest approach from the occurrence of the extremum value of the absolute value of tau is one half of this extremum value. The computer calculates the associated miss distance as the multiplicative product of the average relative velocity of the target with respect to the interceptor times one half of the absolute value of tau at its minimum. These two items of information are used to control the detonation of the warhead for highest lethal effectiveness.

BRIEF DESCRIPTION OF THE DRAWINGS The specific nature of this invention as well as other objects, aspects, uses, and advantages thereof will clearly appear from the following description and from the accompanying drawings, in which:

FIG. 1 is a block diagram of the system and method for computing and detonating said fuze at the'point of closest approach.

FIG. 2 is a plot of the function tau versus time.

FIG. 3 is an approach diagram of the case wherein the target passes in front of the interceptor (early bird case).

FIG. 4 is an approach diagram wherein the target passes behind the interceptor (late bird case).

Note that in FIGS. 3 and 4 the flight paths of the interceptor and target are co-planar and their velocities are constant over the period of interception.

DESCRIPTION OF THE PREFERRED EMBODIMENT In accordance with the state of the art as presented in Merrill I. Skolnik in Radar Systems, McGraw-Hill Co., Inc., New York (I962), chapters 9 and l l, a block diagram illustrating the operation of this system and method is shown in FIG. 1. Referring specifically to FIG. 1, by means of antenna 10 a seeker circuit 20 follows the target 100 and measures the range, r, by well known radar pulse techniques, and stores this value in range measure memory 25. Differentiation unit 26 calls the latest value of the range, r, from memory 25 and differentiates r to obtain a velocity value, dr/dr, and stores this value. Operational divide unit 27 then performs the computation of the ratio as the range to the computed velocity from r called from memory 25 and dr/dr called. from memory of differentiation unit 26. The storage unit 28 stores r- -dr/dt or tau at the instant 'of time t, where i l, 2, 3,...,. Compare unit 29 calls the stored'value of tau at t from storage unit 28 as it receives the value of tau at t, I from divide unit 27 and compares t, 1 to t,- to determine whether I, 1 is greater than 1,. A signal from compare unit 29 is fed to decide (1 -r,) unit 31. If unit 31 receives a r, 1 r, signal, divide by-2 unit 32 divides r, by 2 and feeds a time value, I,,,, is one half of the extremum value of tau.

Tau is a time function that passes through a mathematical extremum (maximum or minimum) when (theta) as shown in FIGS. 3 and 4 is equivalent to 45 .of angular measurement.

After the elapse of the time 9% tau at the extremum values (the extremum value is determined when t, ,t 1 is greater than n), the delay unit 33 initiates fire circuit 34 connected to detonator 35 which causes the fuze to detonate.

The seeker circuit 20 is known in the art and comprises a standard transmitter-receiver (TR) 90, a duplexer (ATR) 92, mixer 91, local oscillator 93, and mixer 91 connected as illustrated in FIG. 1. Also included therein and standard in the art are modulator 94, intermediate frequency amplifier (IF) 95, detector puter and any of the similar circuits described in Skolnik, supra at chapters 3 and 4, are adequate.

In operation, the antenna 10 of the seeker circuit is turned on and off by modulator 94 in such a manner as to generate a desired waveform. The modulator 94 controls the .RF power output of the magnetro 98 which is duplexed to the transmitter by magnetron 92 and TR 90. ATR 92 is the anti-transmit-receive switch and TR 90 is the transmit-receive switch. The mixer 91 mixes the local oscillator signal and input from the TR 90. IF circuit 95 provides an intermediate frequency amplification of the signal from the mixer 91. The deis a standard moving target indicator and range comtector 96 extracts the modulation from the output of the IF amplifier 95. The detector signal is amplified by video amplifier 97 and fed into the range measure memory 25 as a range signal r.

The fuzesystem and method herein described computes the time to the point of closest approach so that the warhead fragments may intercept the target there or at a controlled point elsewhere through delayed or prior detonation. Flight measurements necessary as input data for the fuze include range and range rate, dr/dt, but information involving angular position is not needed. Thus, the required computations are few and simple as indicated in the previous paragraphs describing the block function diagram in FIG. 1. This system and method may be called a extremum tau fuze.

Consider the approach diagram of FIG. 3 which shows the position and velocities of the interceptor l, and the target T. FIG. 3 is the case wherein the target passes in front of the interceptor. The flight paths of the interceptor and target are coplanar and their velocities, V, and V are constant over the period of interception. The velocity of the target relative to the interceptor is V The miss distance is c and the time to go to the point of closest approach (PCA) is t. The range between the interceptor and the target is designated as r. The meaning of the other symbols for angular displacement are apparent from the diagram.

Suppose the fuze measures range, r, and range rate, dr/dt. At long range, 0= 0 so that dr/dt V This information is stored in a memory. As the range decreases the fuze computes the ratio Making use of the fact that the velocity and position triangles are similar triangles, tau is expressed in terms of t and c.

1(tau) (R/V,) (r/R) (V t/V,)sec0 t(sec 0) 1(l +'tan 6) [(1 c /R :(l /V t t (c/V /t The behavior of tau, 1, as a function of time, t, for various ratios ofc to V is shown in FIG. 2. Those branches of the curves that lie in the third quadrant represent the target approaching the interceptor, those in the first quadrant represent separation. The locus of maxima and minima is shown by the dashed line. It can be seen that the equation for this locus is where r is a maximum (or minimum) value of tau, 1', which occurs at t r,,,. This relationship can be confirmed by setting d'r/dt 0 and obtaining t iC/Vg.

7 m, the remaining time to go to the point of closest approach is A timed process can be started to cause the warhead to detonate so that its fragments will strike the target at the point of closest approach.

The distance to be travelled by the warhead fragment is miss distance c. This can be calculated from since V has been stored in the memory. Then the fragment travel time is where V is equal to the fragment velocity. Thus, the time delay for warhead detonation following detection of maximum tau is seconds and the time to go to point of closest approach t, (.02) .01 seconds Suppose the warhead fragment velocity is 7,500 per second. The fuze computes a time delay 1,, .02/2) '1 5,000/7,500 .0033 seconds The warhead is then detonated at 3.3 milliseconds after the occurrence of maximum tau so that the fragments strike the target at the point of closest approach.

The maximum tau system measures range and range rate for ranges less than 50,000 feet. The fuze is usually set for short ranges, to cause detonation when the absolute value of tau drops below some predetermined level such as 3 milliseconds. Continuous monitoring of tau is required in the last tenth of a second in order to observe the extremum. Hence, micro-second computer rates are adequate inasmuch as the times involved are in milliseconds.

Warhead fragment velocity is a predetermined constant which is stored in the computer before launching.

The effect of nonplanar trajectories are minor if the dihedral angle of the trajectory planes is not excessive. Even if the dihedral angle is large, the coplanar assumption is sufficiently valid in the region of closest approach. In the application of this system and method the interceptor missile carries a symmetrical side-spray warhead. Additional delay may sometime be desirable to bring the vulnerable area of an extended target into the fragment pattern, or to permit a tactical ballistic missile to pass the point of closest approach in order to improve the fragment striking angle.

The inventor wishes it to be understood that he does not desire to be limited to the exact detailed construction shown and described, for obvious modifications will occur to a person skilled in the art thereof.

The inventor also wishes it to be understood that the product V r,,, may be utilized as a. means for obtaining the miss distance between said interceptor and target by telemetry or recording means.

What is claimed is:

l. A pulsed radar proximity fuze for obtaining maximum lethal detonation comprising:

a. means for computing the separation distance between said fuze and a target;

b. means for determining the value of the differentiation of said distance;

c. means for computing the ratio of said distance to the value of said differentiation of said distance;

d. means for detecting the extremum value of said rae. means for calculation of one half of said extremum ratio;

f. means for counting time and issuing a first output electrical signal after a preset value is obtained; and

g. means for detecting said first output electrical signal and as a result of said detection issuing a fire signal.

2. The invention of claim 1 further comprising at least one detonator for receiving said fire signal, said detonator being detonated thereby, whereby said fuze is exploded at the point of closest approach to a target.

3. The invention of claim 2 wherein said means for counting time is set to issue a first output electrical signal after a preset value equal to one half of the absolute value of said extremum ratio is obtained.

4. A method for detonating a fuze at the point of closest approach to a target comprising the steps of:

a. computing continually the separation of distance between said fuze and said target and storing said distance;

b. computing continually the value of the differentiation of said distance with respect to time and storing said value;

c. computing continually the ratio of said distance to the value of said differentiation of said distance with respect to time and storing said ratio;

d. detecting the extremum of said ratios;

e. computing one half of the absolute value of said extremum;

f. counting units of time and issuing an electrical output signal after a value of units equal to one half of the absolute value of said extremum is counted; and

g. detecting said output signal and issuing a fire signal to a detonator, whereby said fuze is exploded when the time for travel to the point of closest approach to the target at average velocity transpires. 

1. A pulsed radar proximity fuze for obtaining maximum lethal detonation comprising: a. means for computing the separation distance between said fuze and a target; b. means for determining the value of the differentiation of said distance; c. means for computing the ratio of said distance to the value of said differentiation of said distance; d. means for detecting the extremum value of said ratio; e. means for calculation of one half of said extremum ratio; f. means for counting time and issuing a first output electrical signal after a preseT value is obtained; and g. means for detecting said first output electrical signal and as a result of said detection issuing a fire signal.
 2. The invention of claim 1 further comprising at least one detonator for receiving said fire signal, said detonator being detonated thereby, whereby said fuze is exploded at the point of closest approach to a target.
 3. The invention of claim 2 wherein said means for counting time is set to issue a first output electrical signal after a preset value equal to one half of the absolute value of said extremum ratio is obtained.
 4. A method for detonating a fuze at the point of closest approach to a target comprising the steps of: a. computing continually the separation of distance between said fuze and said target and storing said distance; b. computing continually the value of the differentiation of said distance with respect to time and storing said value; c. computing continually the ratio of said distance to the value of said differentiation of said distance with respect to time and storing said ratio; d. detecting the extremum of said ratios; e. computing one half of the absolute value of said extremum; f. counting units of time and issuing an electrical output signal after a value of units equal to one half of the absolute value of said extremum is counted; and g. detecting said output signal and issuing a fire signal to a detonator, whereby said fuze is exploded when the time for travel to the point of closest approach to the target at average velocity transpires. 